Investigations are proposed to study the effect of organic and inorganic forms of dietray selenium on the initiation, promotion and progression stages of 7, 12 dimethylbenz( )anthracene (DMBA) and 1-methyl-1-nitrosourea (MNU) induced mammary carcinogenesis in the rat. The influence of selenium on the metabolism of DMBA will be studied both in vivo and in mammary gland organ cultures in an attempt to evaluate the mechanism by which tumor initiation is inhibited. The role of pharmocological levels of selenium in blocking macromolecular synthesis will also be investigated. Other work will be undertaken to determine whether selenium kills established tumor cells. Life time studies of the inhibition of mammary carcinogenesis by selenium will be initiated.